1. Field of the Invention
The present invention relates to a laser processing apparatus and a laser irradiation method for crystallizing a semiconductor substrate, a semiconductor film or the like using a laser light or for performing activation after ion implantation, a semiconductor device formed by using the laser apparatus and a manufacturing method therefor, and an electronic equipment using the semiconductor device.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has greatly progressed, and its application and development for active matrix semiconductor display device has been advanced. In particular, since a TFT using a polysilicon film has higher field-effect mobility than a TFT using a conventional amorphous silicon film, it enables high speed operation. Therefore, although the pixel is conventionally controlled on a driving circuit provided outside the substrate, it is possible to control the pixel on the driving circuit formed on the same substrate.
Incidentally, as the substrate used in the semiconductor device, a glass substrate is regarded as important in comparison with a single crystal silicon substrate in terms of the cost. Since a glass substrate is inferior in heat resistance and is susceptible to heat-deformation, in the case where a polysilicon TFT is formed on the glass substrate, laser annealing is used for crystallization of the semiconductor film in order to avoid heat-deformation of the glass substrate.
Characteristics of laser annealing are as follows: it can greatly reduce a processing time in comparison with an annealing method using radiation heating or conductive heating; and it hardly causes thermal damage to the substrate by selectively and locally heating a semiconductor or the semiconductor film.
Note that the laser annealing method here indicates a technique of recrystallizing the damaged layer formed on the semiconductor substrate or the semiconductor film, and a technique of crystallizing the amorphous semiconductor film formed on the substrate. Also, the laser annealing method here includes a technique applied to leveling or surface reforming of the semiconductor substrate or the semiconductor film. A laser oscillation apparatus applied is a gas laser oscillation apparatus represented by an excimer laser or a solid laser oscillation apparatus represented by a YAG laser. It is known as the apparatus which performs crystallization by heating a surface layer of the semiconductor by irradiation of the laser light in an extremely short period of time of about several ten nanoseconds to several hundred microseconds.
Lasers are roughly divided into two types: pulse oscillation and continuous oscillation, according to an oscillation method. In the pulse oscillation laser, an output energy is relatively high, so that mass productivity can be increased assuming the size of a beam spot to be several cm2 or more. In particular, when the shape of the beam spot is processed using an optical system and made to be a linear shape of 10 cm or more in length, it is possible to efficiently perform irradiation of the laser light to the substrate and further enhance the mass productivity. Therefore, for crystallization of the semiconductor film, the use of a pulse oscillation laser is becoming mainstream.
However, in recent years, in crystallization of the semiconductor film, it is found that grain size of the crystal formed in the semiconductor film is larger in the case where the continuous oscillation laser is used than the case where the pulse oscillation laser is used. When the crystal grain size in the semiconductor film becomes large, the mobility of the TFT formed using the semiconductor film becomes high and variation of the TFT characteristics due to a grain boundary is suppressed. Therefore, a continuous oscillation laser is recently attracting attention.
However, since the maximum output energy of the continuous oscillation laser is generally small in comparison with that of the pulse oscillation laser, the size of the beam spot is small, which is about several 10−3 mm2. Accordingly, in order to treat one large substrate, it is necessary to move a beam irradiation position on the substrate upward and downward, and right and left.
In order to move the beam irradiation position upward and downward, and right and left, there are a method in which the position of the substrate is fixed and the irradiation direction of the beam is changed, a method in which the irradiation direction of the beam is fixed and the position of the substrate is moved, and a method in which the above two methods are combined with each other.
When the irradiation direction of the beam is changed, the irradiation angle of the beam with respect to the substrate is changed depending on the position to be irradiated. When the irradiation angle is changed, intensity of the beam returning by reflecting on the substrate, intensity of interference, and the like are changed depending on the position of the substrate. Therefore, it is impossible to uniformly treat the substrate. For example, in the case where the semiconductor film is crystallized by laser irradiation, crystallinity causes a difference depending on the position of the substrate.
On the other hand, in the case where the position of the substrate is moved while fixing the irradiation direction of the beam, the irradiation angle of the beam with respect to the substrate is fixed irrespective of the position of the substrate. Accordingly, the above-mentioned problem is avoided and the optical system is further simplified.
However, there is a loss of time according to the direction change that is a problem in moving the substrate.
FIG. 20 shows a direction in which the irradiation position of the beam on the substrate is moved when the irradiation direction of the beam is fixed and the position of the substrate is moved, by an arrow. In general, in the irradiation of the laser light, after moving the irradiation position in a definite direction, the direction is changed and the irradiation position is moved again in the definite direction. At this time, when the moving speed of the irradiation position is changed depending on the position of the substrate, it is difficult to uniformly treat the substrate. Accordingly, it is necessary to keep the moving speed of the irradiation position constant. Further, in order to change the moving direction of the irradiation position, as shown in portions surrounded by the broken line of FIG. 20, the change is generally conducted when the irradiation position leaves the substrate. After the irradiation position left the substrate, the moving of the substrate is once stopped, and the moving direction of the substrate is changed. Then, after the moving speed of the substrate is increased to the fixed value again, it is necessary to conduct the irradiation to the substrate of the laser light. Consequently, it necessarily takes a predetermined time to change the direction of the substrate, with the result that processing speed of the substrate is lowered.
This is a problem, which is caused also in the case where the irradiation direction of the beam is changed. Since it takes a predetermined time to change the irradiation direction of the beam, it results in lowering the processing speed of the substrate.
In particular, in case of the continuous oscillation laser, differently from the pulse oscillation laser, the size of the beam spot is originally small. Thus, processing efficiency is poor and it is an important object to improve the processing speed of the substrate.